How does photorespiration affect the efficiency of photosynthesis?

Photorespiration reduces the efficiency of photosynthesis by diverting energy and carbon away from the production of glucose.

Photorespiration is a process that occurs in plants when the concentration of carbon dioxide is low and the concentration of oxygen is high. This typically happens in hot and dry conditions when plants close their stomata to prevent water loss. When the stomata are closed, carbon dioxide cannot enter the leaf and oxygen cannot exit. As a result, the concentration of carbon dioxide decreases and the concentration of oxygen increases inside the leaf. This causes the enzyme Rubisco, which normally catalyses the reaction between carbon dioxide and Ribulose Bisphosphate (RuBP) in the Calvin cycle of photosynthesis, to bind with oxygen instead of carbon dioxide.

When Rubisco binds with oxygen, it initiates a process called photorespiration. This process uses ATP and NADPH, which are energy-rich compounds produced in the light-dependent reactions of photosynthesis, but does not produce any glucose. Instead, it produces a two-carbon compound called glycolate, which is then converted into glycine and serine in the peroxisomes and mitochondria. These amino acids are then transported back to the chloroplasts where they are converted back into RuBP. This entire process is energy-intensive and does not contribute to the production of glucose, which is the main goal of photosynthesis.

In addition, photorespiration also results in the release of carbon dioxide. This is problematic because carbon dioxide is a crucial substrate for the Calvin cycle. When it is released through photorespiration, it is not available for the Calvin cycle, which further reduces the efficiency of photosynthesis.

In summary, photorespiration reduces the efficiency of photosynthesis by diverting energy and carbon away from the production of glucose. It uses up ATP and NADPH, releases carbon dioxide, and does not contribute to the production of glucose. This is why plants have evolved various strategies to minimise photorespiration, such as the C4 and CAM pathways, which allow them to efficiently carry out photosynthesis even in hot and dry conditions.